A conventional hydraulic system includes a pump that draws low-pressure fluid from a tank, pressurizes the fluid, and makes the pressurized fluid available to multiple different actuators for use in moving the actuators. In this arrangement, a speed and/or force of each actuator can be independently controlled by selectively throttling (i.e., restricting) a flow of the pressurized fluid from the pump into and/or out of each actuator. An alternative type of hydraulic system is known as a meterless hydraulic system, which generally includes a pump connected in closed-loop fashion to one or more actuators. During operation, the pump draws fluid from one chamber of the actuator(s) and discharges pressurized fluid to an opposing chamber of the same actuator(s). To move the actuator(s) at a higher speed, the pump discharges fluid at a faster rate. A common type of actuator is a double-acting cylinder having a single rod that moves a piston between a “rod end” of the cylinder that is opposite a “head end” of the cylinder.
One problem with meterless hydraulic systems involves passing fluid between the head end and rod end of a double-acting cylinder. Because the volume of the rod end is reduced by the volume of the rod, the head and rod ends consume and discharge different volumes of fluid for a given movement of the cylinder, which can lead to starving or stalling of the pump. Also, when an associated load of a work tool attached to the cylinder suddenly changes directions, the pump displacement must be adjusted to avoid creating velocity discontinuities of the cylinder movement, which can cause the system to operate in a jerky manner. Further, unintended movements (e.g., bouncing) of the associated load of the work tool may create fluid pressure oscillations that can travel back to the pump in a meterless system. These oscillations may also cause the pump to behave in a jerky manner.
One attempt to accommodate a difference between the head end volume and the rod end volume of a hydraulic cylinder is described in U.S. Pat. No. 6,912,849 B2 (the '849 patent) that issued to Inoue et al. on Jul. 5, 2005. In the '849 patent, a closed-loop hydraulic system is described. The hydraulic system includes a pump that has a first port connected to the head end of a hydraulic cylinder, a second port connected to the rod end of the hydraulic cylinder, and a third port connected to a tank. The pump is driven by an electric motor, which controls the speed, direction, and discharge rate of the pump. When rotated in a first direction, fluid from the head end of the cylinder is drawn into the pump, apportioned, and expelled to the rod end of the cylinder and to the tank. When rotated in the opposite direction, fluid from the rod end and from the tank is drawn into the pump, combined, and expelled to the head end of the cylinder. When braking is applied to slow the pump, energy is recovered as electricity by the electric motor.
Although somewhat effective at accommodating the difference between head end and rod end volumes of a hydraulic cylinder, the system of the '894 patent may not be optimum. Specifically, the '894 system may still operate in an overly jerky manner, which may result in a shortened lifespan of the pump and discomfort to the operator of an associated machine. Further, the pump of the '894 system may be large and therefore less efficient. The '894 system may also experience pressure losses during retraction strokes when fluid from the head is directed to the tank, thereby further reducing the system's efficiency.
The hydraulic system of the present disclosure is directed toward solving one or more of the problems set forth, above and/or other problems of the prior art.